It is well known to those skilled in the art that gelled or crosslinked water-soluble polymers are useful in enhanced oil recovery and other oil field operations. They have been used to alter the permeability of underground formations in order to enhance the effectiveness of water flooding operations. Generally, polymers along with an appropriate crosslinking system are injected in an aqueous solution into the formation. The polymers then permeate into and gel in the regions having the highest water permeability.
Because of environmental concerns as well as cost for disposing of a produced brine which is defined as the brine co-produced with oil and gas, it is desirable to utilize the produced brine as the aqueous solution used for the polymers and appropriate crosslinking system described above. Use of produced brines eliminates not only the cost associated with acquiring and pre-treating fresh water for use as the aqueous solution but also the disposal cost for the produced brine. Most produced brines are known to be hard brines, i.e., those having a salinity of greater than 2% total dissolved solids, basically inorganic salts. Chromium(III) carboxylates such as, for example, chromium acetate are the only known crosslinkers which can be used to produce stable gels in produced brines for near-wellbore treatment. See for example R. D. Sydansk, Acrylamide-Polymer/Chromium(III)-Carboxylate Gels for Near Wellbore Matrix Treatments, Proceedings SPE/DOE Seventh Symposium on Enhanced Oil Recovery (1990). Although a chromium(III) salt is not as toxic as a chromium(VI) salt, it is not an environmentally desirable compound and its use may require additional costs to assure the integrity of the injection wells to avoid contamination of ground water sources.
Furthermore, most gelling compositions form permanent gels, i.e., gels that are stable for longer than about 360 days. For many oil field applications, permanent gels are undesirable. For example, in formations where there are a lot of casing failure due to subsidence, gelling compositions are injected between tubing and casing to block water leakage into the well. If permanent gels were formed between the tubing and casing, it could be very difficult to remove the tubing when needed. Temporary gels which disappear by themselves in the formation in less than about 180 days would seem very desirable for this type of application.
For selective stimulations, a very high viscosity gel may be placed in the casing across a lower open interval while fracturing an upper interval. Currently sand is placed in the casing in the lower interval to allow selective stimulation of the upper interval. Use of a temporary gel instead of sand eliminates the need for sand cleanout after the stimulation. Temporary gels can also be placed in high permeability channels before injecting the fracturing fluids. After the gels are set, the fracturing fluids can be injected which will go into the lower permeability zones and fracture them. This will result in better stimulation.
An additional example of using temporary gels is the use of these gels in workover of gas wells. A temporary gel can be placed in the gas producing zone to protect it against formation damage caused by completion fluids.
Additionally, temporary gels can be placed in high permeability channels before injecting acid which would help to clean up the lower permeability zones. Temporary gels can also be used as "Gel Pig" for cleaning the pipelines. The advantage of temporary gels over permanent gels is that if the gels are designed properly to break down at the end of cleaning process, it would be easier to handle the decomposition product which is liquid as compared to solid gels.
Thus, it would be a significant contribution to the art if a composition that forms a temporary gel in subterranean formations can be developed.